EP0288111B1

EP0288111B1 - Symmetrical electrical snubber circuit

Info

Publication number: EP0288111B1
Application number: EP88200696A
Authority: EP
Inventors: Hendrik Johan Mezger; Jacob Cornelis Van Strien
Original assignee: Holec Systemen en Componenten BV
Current assignee: Holec Systemen en Componenten BV
Priority date: 1987-04-16
Filing date: 1988-04-11
Publication date: 1992-07-01
Anticipated expiration: 2008-04-11
Also published as: ATE77904T1; FI881742A0; DE3872434T2; NO172826B; FI87956B; FI881742A; NL8700918A; DK208988D0; FI87956C; EP0288111A1; DE3872434D1; ES2034155T3; NO881655D0; DK208988A; NO881655L; NO172826C

Abstract

Symmetrical electrical snubber circuit for protecting at least two series-connected switch members (S1; S2) against excessive voltage and current increase during switching. The voltage increase over each of said switch members is limited by a series circuit of a capacitor and an element having diode action (D1, C1; D2, D2) connected in parallel with said switch members. The magnitude of the DC supply voltage is effectively limited by a further series circuit of a capactior and an element having diode action (D3, C3; D4, C4) connected with said switch members. A freewheeling diode (D5, D6) is connected in parallel with each of said switch members. By incorporating a coil (L1, L2) in the supply lines to the switch members, the current increase during switching-on of a switch member is limited. Like this, a symmetrical snubber circuit which is as low in stray inductance as possible is obtained. The energy stored in the coils (L1, L2) as well as the energy stored in the capacitors (C1; C2) being transferred to the further capacitors (C3; C4) due to resonant action. The energy in these further capacitors can then be discharged with the aid of energy transport means (ET). The snubber circuit according to the invention is in essence loss-free.

Description

The present invention relates to a symmetrical electrical snubber circuit for protecting at least two series-connected switch members of which the ends which are not connected to each other form a positive and a negative terminal, respectively, for connecting, via at least one coil, a direct voltage, at least one first element having diode action connected in the reverse direction and a series circuit of at least one capacitor and at least one second element having diode action connected in the forward direction relative to the terminals for direct voltage being connected in parallel to each of the switch members respectively, said at least one capacitor of each series circuit being connected to the mutual connection point of the switch members, and means for discharging the energy stored in the capacitors being provided.
A symmetrical electrical snubber circuit of this type is known from German Patent Application DE-A1-3,521,983.
In this known snubber circuit, which is of the so-called direct or switch snubber type, the respective capacitors limit the voltage increase (dv/dt) across the switch members during switching-off, i.e. the transition to which they become non-conductive, and the coil results in a limitation of the current increase (di/dt) through the switch members during switching-on, i.e. the transition to which they become conductive. The first element, connected anti-parallel to each switch member and having diode action fulfils the "free-wheeling diode" function during inductive loading and in switch members such as gate turn off (GTO) thyristors at the same time prevents passage of current in the nonpermissible direction.
Besides this direct or switch snubbering, German Patent Application DE-A1-2,639,589 furthermore discloses the principle of indirect or load snubbering. In this, the series circuit of the at least one capacitor and the at least one second element having diode action is connected in parallel to the load, in contrast to the switch snubbering.
The at least one coil for limiting the current increase is located in this arrangement in the supply line between the series circuit and the load.
Depending on the stray inductance unavoidably present in the supply and in the connections of the circuit, the stray self-inductance of the electronic components of the circuit and the rate of the current change during switching-off of a switch member, a voltage overshoot occurs in the direct voltage of the supply and a voltage peak across the switch member itself.
The stray inductance of the supply and of the connecting lines thereof to the switch members can be imagined as being replaced by an equivalent stray inductance in the supply circuit. In switch snubbering, this equivalent stray inductance influences the voltage overshoot, whereas in the case of load snubbering it mainly determines the voltage peak across the switch members themselves. The stray self-inductance of the series circuit of the one capacitor and the one second element having diode action as well as the stray inductance of their connecting lines by contrast determines, in the case of switch snubbering, the voltage peak across the associated switch member and in load snubbering, it determines the voltage overshoot of the supply voltage. The energy stored in the at least one coil for limiting the current increase results in an increase in the voltage overshoot in the direct voltage of the supply.
In the case of switch snubbering, the series circuit of the at least one capacitor and second element having diode action may be arranged as closely as possible to the switch member in order to minimize the voltage peak across it. The equivalent stray inductance in the supply circuit is more difficult to control in practice so that the voltage peak across the switch member is larger in the case of load snubbering than in the case of switch snubbering. The converse applies for voltage overshoot.
In particular when using semi-conductor switches such as GTO thyristors, field-effect transistors (FET's) and bipolar transistors, such a voltage peak or voltage overshoot across a non-conductive switch member may lead to breakdown thereof as a result of which it may become defective. Particularly in the case of load snubbering, above certain switch-off currents it becomes impossible, as a consequence of the high switch-off losses in the switch members, to use, for example, GTO thyristors. Switch snubbering is therefore to be preferred.
German Patent Application DE-2,128,454 discloses in fig. 7 a symmetrical switch snubber circuit, wherein the means for discharging the energy stored in the associated capacitor on switching a switch member from the switched-on to the switched-off state consist of a resistor connected in parallel to said capacitor. During repeated switching-on of the associated switch member, this energy is converted to heat in this resistor. During switching-off of the switch members, the energy stored in the at least one coil will also be converted to heat in these parallel resistors.
The discharge time of the capacitors is essentially determined by the associated resistor in which they discharge. This resistor cannot be chosen to be too small because this reduces the voltage increase limiting operation of the associated capacitor. On the other hand, the resistor cannot be chosen to be too large because discharge then takes too long as a result of which the frequency with which the switch member can be switched is reduced. Since the switch members are only optimally protected by the capacitors against too high a voltage increase when they are discharged.
In the circuit known from DE-A1-3,521,983 already mentioned in the preamble, instead of a resistor, a series circuit of a diode element and a voltage source allowing back transfer of electric energy is disclosed, for discharging the energy stored in the associated capacitor and the current increase limiting coil essentially without loss of energy. However, it is mentioned that said voltage source may be replaced by a resistor or other electric energy consumable element.
Although DE-A1-3,521,983 discloses a snubber circuit for a symmetrically arranged pair of switches, the circuit itself is composed of identical protective circuits, operating individually for an associated switch.
The invention now has the object of providing an essentially loss-free symmetrical snubber circuit low in stray inductance with which the voltage overshoot in the supply voltage and the voltage peaks across the switch members are at the same time effectively reduced.
This is achieved according to the present invention by providing a further series circuit of at least one further capacitor and at least one third element having diode action connected in the forward direction relative to the terminals for direct voltage, and being connected, respectively, between the connecting point of said at least one capacitor and said at least one second element having diode action of the series circuit associated with the one switch member and the terminal for direct voltage associated with the other switch member, all this in a manner such that a snubber circuit which is as low in stray inductance as possible is obtained, and wherein the means for discharging the stored energy being connected to the respective further capacitors.
The means for discharging the stored energy can be implemented in a known manner using at least one switch member, one element having diode action and one coil, with which the energy stored in the further capacitors can be put to useful application or can be returned to the supply source.
Detailed consideration of the operation of the circuit according to the invention in comparison with the circuit known from DE-A1-3,521,983 shows that in the circuit according to the invention the capacitors of a series circuit connected in parallel to the switch members are simultaneously active during transfer of the current through the load during switching-off of a switch member. These capacitors may therefore each have smaller dimensions than in a comparable application with the known circuit and therefore have a smaller stray self-inductance.
The invention consequently provides a symmetrical snubber circuit low in stray inductance which on the one hand provides protection, via the capacitors connected in parallel to the switch members, against an excessive voltage increase over the switch members themselves and on the other hand effectively limits, via the further capacitors, the magnitude of the direct voltage of the supply across the switch members. By incorporating the at least one coil in the connection to the switch members between the supply source and the associated terminal for direct voltage of a switch member an effective protection against an excessive current increase during switching-on is obtained, the energy stored in this at least one coil as well as the energy stored in the capacitors connected in parallel to the switch members being transferred to the further capacitors during switching due to resonant action, so that it is not necessary to incorpate any resistors in the circuit in which the energy is converted to heat. The energy stored in these further capacitors can then be returned, with the aid of the energy transport means, in a useful manner to the direct voltage supply source. The snubber circuit according to the invention is thus in essence loss-free.
In order to be able to store both the energy of the capacitors connected in parallel to the switch members as well as the energy of the at least one current increase-limiting coil present in the circuit, the further capacitors have a larger value than the capacitors connected in parallel to the switch members. For considerations of symmetry, a current increase-limiting coil is preferably incorporated in the two connections of the direct voltage to the terminals associated with the switch members.
It is mentioned that the said patent specification WO-A-8,400,858 discloses a non-symmetrical loss-free snubber circuit for a series connection of two switch members, but in which a capacitor is connected in parallel expressly only to just one switch member and only one further capacitor is applied for receiving the energy stored in the current increase-limiting coil and the first-mentioned capacitor. It is attempted to protect both switch members with this non-symmetrical snubber circuit which in principle is a combination of switch snubbering and load snubbering. A larger voltage peak will therefore occur over one of the switch members than is the case with the symmetrical circuit according to the invention.
The energy build-up during switching over of the switch members is distributed over two capacitors in the symmetrical snubber circuit according to the invention while all the energy has to be stored in one larger capacitor in the non-symmetrical snubber circuit.
The snubber circuit according to the invention furthermore has the advantage that as a result of the one switch member becoming non-conductive the capacitor connected in parallel to the other switch member is discharged via the load connected to the connecting point of the two switch members, so that the discharging current will not flow through the associated switch member when the latter again becomes conductive and the frequency at which the switch members can be switched may be larger because the associated capacitors are discharged earlier.
Due to its symmetry, the subber circuit according to the invention is particularly suitable for application in combination with very high-speed switching semi-conductor elements such as GTO power thyristors or power transistors.
Further advantages and uses of the symmetrical snubber circuit according to the invention in, inter alia, an inverter having three output voltage levels and a three-phase circuit as well as the operation thereof will be explained in detail in the text which follows with reference to exemplary embodiments shown in the drawings, in which:

Figure 1 shows an example of a simple switch snubber circuit for protecting one switch member.
Figure 2 shows an example of a simple load snubber circuit for protecting one switch member.
Figure 3 shows, with solid and broken lines, the course of the voltage variation across the switch member of Figure 1 and Figure 2, respectively during switching-off thereof.
Figure 4 shows the known symmetrical switch snubber circuit, built up of two circuits according to Figure 1.
Figure 5 shows a simple circuit for limiting the voltage overshoot caused by the stray inductance in the circuit and in the one or the two current increase-limiting coils.
Figure 6 shows a preferred embodiment of the symmetrical snubber circuit according to the invention.
Figure 7 shows a possible embodiment of a circuit for discharging the energy stored in the further capacitors.
Figures 8-10 show the current flow through the circuit according to Figure 6 in various states of the switch members.
Figure 11 shows a preferred embodiment of a symmetrical three-phase snubber circuit according to the invention.
Figure 12 shows a preferred embodiment of a snubber circuit according to the invention in a circuit for obtaining an output voltage of three levels.

Figure 1 gives an example of a so-called non-loss-free switch snubber. The voltage source U_DC forms the supply of the circuit and is often also represented as a capacitor (not shown). The load Z_L is connected in series with the switch member S₁ to the voltage source. The equivalent stray inductance L_s of the supply lines to the circuit and the stray in the supply U_DC is also incorporated in this series circuit. A freewheeling diode D_L is connected in the reverse direction relative to the supply voltage parallel to the load Z_L which is assumed to be inductive. The snubber circuit consists of a parallel circuit of a diode D₁ and a resistor R₁ in series with a capacitor C₁. The snubber circuit is connected in parallel to the switch member S₁. For the sake of clarity, the switch member is drawn as a mechanical switch but, as already mentioned in the introduction, this may of course also be a semiconductor switch such as a GTO thyristor, FET or bipolar transistor. The circuit operates as follows.
When the switch member S₁ is switched on, i.e. is in its conductive state, the capacitor C₁ will be totally discharged via the resistor R₁. The energy stored in the capacitor C₁ is then dissipated as heat in the resistor R₁. When the switch member S₁ switches off, i.e. transites into the non-conducting state, the voltage across the capacitor C₁ will thus be equal to zero.
As a result of the inductive character of the load Z_L, the behaviour of which at the moment of switching off can be approximated by a current source, current will remain flowing in the circuit when the switch member S₁ is switched off. This means that the current i which flowed first through the switch member S₁ then has to be taken over by the diode D₁ in series with the capacitor C₁. The voltage waveform which is produced across the switch member S₁ during switching-off thereof is represented with a solid line in Figure 3. At time t₀, the switch member begins to switch off and at time t₁ it is completely switched off. Directly after switching-off, there is a voltage peak with a maximum value VDP over the switch member. Because the voltage across the capacitor C₁ cannot abruptly change, it is necessary to look for the cause of this voltage peak in the stray inductance in the circuit formed by S₁-D₁-C₁. The magnitude of the maximum value V_DP of the voltage peak is, besides the magnitude of this stray inductance, also dependent on the rate at which the switch member S₁ switches, i.e. the current change per unit time di/dt. When the current is taken over by the capacitor C₁ and the diode D₁ the voltage U_S1 over the switch member S₁ will increase linearly up to the supply voltage U_DC. At that instant, the current through the load Z_L is taken over by its freewheeling diode D_L and an overshoot occurs in the voltage U_S1 because the energy stored in the stray inductance L_s, having a magnitude of 1/2 L_si², is transported to the capacitor C₁. The current through the inductive load Z_L can continue to flow via its freewheeling diode D_L. The capacitor C₁ is discharged again via the resistor R₁ down to the supply voltage U_DC, as can be seen by the solid line in Figure 3, and thereafter remains charged up at this value.
Because there is always stray self-inductance present in the circuit S₁-D₁-C₁, a voltage peak resulting in switching-off losses will always occur for a high di/dt during switching-off of the switch member S₁. In order to keep these switching-off losses low it is therefore important to keep the stray inductance in these circuits as small as possible. In order, furthermore, to limit the voltage overshoot over the switch member S₁ as much as possible it is necessary to keep the equivalent stray inductance L_s as small as possible.
In order to limit the current increase during switching-on of the switch member S₁ a coil in series with the supply of the circuit is frequently incorporated in practice, for example the coil L₁ shown by dotted lines in Figure 1. This coil has a freewheeling path via a series circuit, connected in parallel thereto, consisting of a diode D_C, in rest direction relative to the supply voltage, and a resistor R_C. Due to the resistor R_C the current through the coil L₁ will decrease during recovery. The voltage over the coil then increases the voltage overshoot in the direct voltage of the supply. The coil L₁ is not necessary for every application. This depends, inter alia, on the speed with which the switch member S₁ switches.
Figure 2 gives an example of a so-called non-loss-free load snubbering. The components given with the same indices correspond to those shown in Figure 1. The snubber circuit D₁, C₁, R₁ is connected in parallel to the inductive load Z_L with freewheeling diode D_L. The circuit operates as follows.
When the switch member S₁ is switched on, the capacitor C₁ is charged up via the resistor R₁. At the same time, energy is dissipated in R₁. The voltage waveform U_S1 which is produced across the switch member S₁ during switching-off thereof is shown with a broken line in Figure 3. At the time t₀, S₁ begins to switch off and at t₁ it is completely switched off. Directly after switching-off, a voltage peak occurs again over the switch member but now with a maximum value Vʹ_DP. The magnitude thereof is determined by the stray self-inductance in the circuit formed by S₁-C₁-D₁-L_s, Ls again representing the equivalent stray inductance of the supply and the connecting lines to the load and the switch member, and by the di/dt during switching-off of S₁. In contrast to the circuit of Figure 1, L_s is now located in the circuit which determines the voltage peak as a result of which Vʹ_DP is larger than V_DP, with the switching-off conditions being the same as in the circuit of Figure 1, as shown in Figure 3. When the switch member is switched off and the current i through the load is taken over by C₁-D₁ the voltage US₁ increases linearly to the supply voltage U_DC after the voltage peak. At that instant, the current through load Z_L is taken by its own freewheeling diode D_L. This circuit shows very little voltage overshoot because the stray self-inductance in C₁-D₁-D₂ is low. Although not shown in the drawing, a current increase-limiting coil L₁ with a series circuit, connected in parallel thereto, of a resistor R_C and diode D_C connected in the reverse direction relative to the supply voltage can also be incorporated in this circuit. This current increase-limiting coil L₁ should be incorporated in this case in the connecting line between R₁-D₁ and the load Z_L.
On comparison of the load snubber with the switch snubber it is noteworthy that the former generally has a larger voltage peak which determines the eneoff thereof. This larger voltage peak is caused not only by the energy stored in the equivalent stray inductance L_s being dissipated in the switch member but also by the physical dimensions of, in particular, the connecting lines between the snubber circuit and the load being larger than in the switch snubber, which results in a larger stray inductance in the circuit. Especially in the case of high-speed switch members such as GTO-thyristors, in which a high di/dt occurs, as already mentioned in the introduction, it becomes impossible to use load snubbering above certain switch-off currents as a result of the high switch-off losses in the switch members.
In view of the limitations imposed by load snubbering on the magnitude of the current to be switched off, particularly with high-speed switching switch members, attention in the text which follows will be focused also on improving the switch snubber.
The above-described snubber circuit may also be doubly designed, as a bridge circuit, so that the load can be applied alternately to the positive and to the negative terminals of the supply source U_DC for, for example, inverter operation. The circuit is then as shown in Figure 4 and corresponds to the symmetrical switch snubber of the state of the art. In this circuit, two switch members S₁ and S₂ have been incorporated in a series circuit, each with its own snubber circuit. The coil L₁ again serves for limiting the current increase during switching-on of a switch member. The operation of the circuit is in principle the same as with a single switch member, with the proviso that the switch members are switched alternately. The connecting point of the two switch members forms the output of the circuit to which a load can be connected. The freewheeling diode D_L shown in Figure 1 is replaced, in the circuit of Figure 4 as shown, by a diode, D₅ and D₆ respectively, connected in parallel to each switch member.
The biggest disadvantage of the above-described single and double snubber circuits is the fact that the energy stored in the capacitors C₁ or C₂ is dissipated in a resistor during switching-on of the associated switch member. The energy stored in the coil L₁ is dissipated in the resistor R_C during switching-off of S₁ or S₂. As already indicated in the introduction, the value of the resistors R₁, R₂ cannot be chosen to be too small in view of an excessive load on the associated switch member during switching-on thereof. An excessively high value of the said resistors means that the capacitors are discharged too slowly, which reduces, as mentioned, the frequency at which the switch members can be switched. This circuit furthermore possesses no protection against the voltage overshoot caused by stray inductance L_s and the resistor R_C. All the energy stored in the stray inductance L_s and the voltage drop across the resistor R_C together form the voltage overshoot which may result in breakdown across a switched-off switch member, in particular in, for example, semiconductor switches, with all the disadvantageous consequences thereof.
A simple and expedient manner of limiting voltage overshoot is the use of an overvoltage protection or so-called clipping circuit. An example of a very simple clipping circuit is shown in Figure 5. In parallel to the series circuit of the load Z_L and the switch member S₁, there is connected a further series circuit comprising a capacitor C₃ and a diode D₃ connected in the forward direction relative to the supplying direct voltage U_DC. This series circuit forms the clipping circuit.
The capacitor C₃ of the clipping circuit, is relatively large relative to the capacitor C₁. As a result of the diode D₃, the capacitor C₃ remains at a constant voltage U_c which is larger than U_DC. When the voltage over the clipping circuit becomes larger than the voltage over the capacitor C₃ the diode D₃ will start to conduct so that the voltage across the load and the switch member is kept at the value U_c. This means that the magnitude of the voltage U_AB across the switch member is limited by means of the clipping circuit. The energy stored in the stray inductance L_s and the coil L₁ is then supplied to the capacitor C₃, as a result of which L₁ does not have to be provided with a freewheeling circuit. The energy stored in the capacitor C₃ must be discharged in one manner or another, for example by being returned to the direct voltage source. The means for discharging this energy are not shown in Figure 5.
In the circuit according to the invention, at least two of these clipping circuits and a symmetrical snubber circuit according to the state of the art are then integrated in a manner such that the energy stored in the capacitors associated with the switch members and the energy of the used current increase-limiting coil(s) are supplied, during switching, to the capacitors of the respective clipping circuits, as a result of which there is no need to use a resistor in which this energy is dissipated. This results in an in essence loss-free snubber circuit. Because there are always two circuits active in the circuit according to the invention during switching over of a switch member an effective reduction of the influence of the unavoidable stray self-inductance is obtained. A preferred embodiment of the circuit according to the invention is shown in Figure 6
In a manner which is as low in inductance as possible, a freewheeling diode D₅, D₆ and a series circuit consisting of a diode and a capacitor, D₁, C₁ and D₂, C₂, respectively, are connected in parallel to each switch member S₁, S₂. The capacitors C₁ and C₂ are therefore each connected by one end to the connecting point C of the two switch members S₁, S₂. A clipping circuit consisting of a diode D₃ and a capacitor C₃ is connected between the connecting point of the diode D₁ and the capacitor C₁ and the negative terminal B. Likewise, a clipping circuit consisting of a capacitor C₄ and a diode D₄ is connected between the terminal A and the connecting point of the series circuit consisting of the capacitor C₂ and the diode D₂. In order to keep the stray inductance in the circuit as low as possible, the diverse diodes and capacitors should be connected physically as close as possible to the respective switch members. A small stray inductance is absolutely necessary when high currents need to be switched off at high speed.
Means ET (energy transport) are connected to the capacitors C₃ and C₄ for discharging in a useful manner the energy stored in these capacitors. The means ET can be implemented in various ways, for example as shown in Figure 7. The supply circuits furthermore include two current increase-limiting coils L₁ and L₂. Use of more than one current increase-limiting coil depends on the use to which the circuit is to be put, the rate at which the switch members switch on and the type of the load which is connected to the connecting point C of the two switch members S₁ and S₂. For a satisfactory operation of the circuit it is essentially important that the current increase-limiting coil(s) L₁, L₂ is (are) connected at one end to the direct voltage of the supply and at the other end to the further capacitors C₄, C₃ respectively.
The diodes and capacitors C₃, D₃ and C₄, D₄ do not necessarily have to be connected in the sequence shown in Figure 6 to the terminals B, A respectively for direct voltage. The diode D₃ and the capacitor C₃ may for example be exchanged in position, as can the diode D₄ and the capacitor C₄. The capacitors C₃ and C₄ must of course have a capacitance such that they are capable of storing the energy stored in the capacitors C₁ and C₂ as well as in the current increase-limiting coil(s) L₁, L₂.
The elements shown in Figure 6 with a diode symbol do not necessarily have to be diodes but may be substituted by any other element having diode action, such as, for example, a thyristor acting as a diode. If desired, it is also possible to connect a plurality of elements in parallel or in series, which also applies to the capacitors shown. The switch members S₁ and S₂ may furthermore consist of groups of several parallel-connected switch members or may be designed as a so-called two-way switch. The switch members may furthermore form part of a so-called inverter circuit, chopper circuit, DC/DC converters and the like
Figure 7 shows a possible embodiment of the energy-discharge means ET, the energy stored in the capacitors C₃, C₄ being returned periodically to the direct voltage. A resonant circuit is formed by means of the switch members S₄, S₅ which of course are preferably also controlled semiconductor switches, as a result of switching-on thereof with C₃, C₄ respectively and the coil L₃, the energy from the capacitors being transferred to the coil. By switching the relevant switch member off again at the instant at which the energy is deposited in L₃, the resonant circuit is interrupted and L₃ supplies its energy via the diodes D₇, D₈ back to the supply source U_DC, whereafter the cycle may be repeated. Instead of supplying back to U_DC, it is also possible to supply an arbitrary direct current load in this manner, for example a fan for forced cooling of the circuit.
The operation of the circuit according to the invention is now illustrated with reference to Figures 8-10. In these, the output current I_o through the load Z_L is shown in only one direction. The operation in the other direction is conformable because of the symmetry of the snubber circuit. As already indicated, the load Z_L is assumed to be highly inductive in nature. The time constant of the load Z_L is assumed to be so large that the load during the switching can be regarded as being virtually a current source. For the sake of simplicity, the energy-discharge means ET have been omitted.
Imagine a situation in which the switch member S₁ is switched on and the switch member S₂ is switched off. The capacitors C₃ and C₄ are charged up to a voltage U_c just like the capacitor C₂ which is charged up via S₁ and D₂ to the voltage U_c, U_c being larger than U_DC. Capacitor C₁ is discharged.
The switch member S₁ will subsequently be switched off and the switch member S₂ switched on, it being assumed that S₂ does not switch on before S₁ changes to the non-conducting state. In order to achieve this, there will be a short delay time between switching-off of S₁ and switching-on of S₂. Due to this delay time there may be two different possibilities for the output voltage to change sign, namely

a. The inductive current through the load Z_L is so large that the output voltage changes sign during the delay time, or
b. S₂ switches on before the output voltage has become low.

It is assumed that the switch member S₁ switches very quickly (ideal).
Let us now look at the situation as indicated under a. After the switching-off of S₁, a current flow as represented by the thick solid lines in Figure 8 will be obtained after a short time in the snubber circuit. Two current paths, namely a first current path, very low in stray self-inductance, via D₁-C₁ and a second current path via C₄-D₄-C₂ form in the snubber circuit, the currents through the load Z_L flowing back to the voltage supply. In this phase the capacitor C₁ is charged up and thus S₁ is protected against an excessive voltage increase, and capacitor C₂ is discharged via load Z_L. The voltage across the load Z_L decreases.
As soon as the voltage across the load Z_L becomes zero, freewheeling diode D₆ becomes conductive. This results in the current due to the energy stored in the load Z_L continuing to flow via this diode D₆, as indicated by the thick broken line in Figure 8. The energy stored in coils L₁ and L₂ will then be stored resonantly in the capacitor C₁ and the capacitor C₄. The resonant circuits therefore are formed by

${L₁-[(D₁-C₁)//(C₄-D₄C₂)]-(D₆//Z}_{L})-L₂$

"//" indicating a parallel connection. In order to obtain these resonant circuits it is essential that the current increase-limiting coils L₁ and/or L₂ are incorporated in the circuit in the position shown in the figures.
As soon as the voltage over the capacitor C₁ becomes larger than the voltage over the capacitor C₃, and respectively the voltage over the capacitor C₂ becomes equal to zero, the diodes D₃ and D₂, respectively, become conductive the voltage U_AB over the series circuit of the switch members S₁, S₂ is limited to the value Uc because the capacitors C₃ and C₄ are chosen so large that during this phase the voltage over these capacitors may be assumed to be constant and thus the voltage over the series circuit of the switch members is constant. The current flow in this situation is shown in Figure 9. During this phase, the energy from the coils L₁ and L₂ is stored further in the capacitors C₃ and C₄.
As soon as the current through the coils L₁ and L₂ is zero the voltage U_AB will return to the value U_DC and the circuit comes to rest. In this situation the capacitor C₁ is charged to U_c and the capacitor C₂ discharged. After the said delay time has elapsed the switch member S₂ will switch on, as a result of which the current which may still flow through the freewheeling diode D₆ of the load Z_L, depending on the type of switch member, will then start to flow through S₂ or remain flowing through D₆.
Let us now look at the situation described under b. The voltages across the capacitors before the switch member S₁ switches off are the same as in the above-described situation. A current flow as shown by the thick solid lines in Figure 8 will also be obtained in this case when S₁ is switched off. When the switch member S₂ switches on a current as indicated by the thick broken line in Figure 8 will start to flow through it. A resonant phenomenon now occurs by which the capacitor C₁ is further charged up, and the capacitor C₂ further discharged. As soon as the voltage over C₁ becomes larger than the voltage across the capacitor C₃, respectively the voltage across C₂ becomes equal to zero, the diodes D₃ and D₂ become conductive and a current flow is produced as shown in Figure 9. The circuit will come to rest in the same manner as described for the situation a.
When the circuit has come to rest in the above-mentioned situations a and b as described, switching off of the switch member S₂ will cause no change because the current flowing through the load Z_L will then be taken over by the freewheeling diode D₆.
When switch member S₁ is thereafter switched on again the voltage U_AB becomes 0 volt. The current through S₁ will increase. The gradient of the current increase is limited by L₁, L₂ to (di/dt)S₁ = U_DC/L₁,L₂. When the current through S₁ is equal to I_o the diode D₆ will start to block. A resonant circuit has then been formed by L₁-S₁-[(C₁-D₃-C₃)// (C₂-D₂)]-L₂. As a result C₁ is discharged to 0 volt and C₂ charged up to U_c. At the instant at which this situation is achieved the voltage U_AB is limited by (D₁-D₃-D₃)// (C₄-D₄-D₂) to U_c (Figure 10). The surplus energy from L₁, L₂ is stored in C₃, C₄. The current through L₁, L₂ is equal to I_o.
When, as in practice, the switch members form part of, for example, a three-phase inverter circuit as shown in Figure 11 in which the right-hand index of the numerals of the elements corresponds to the index of the circuit according to Figure 6, a load will be connected alternately between points (AC)_i and (BC)_i i = 1, 2, 3 ... depending on the state of the other switch members. This means that a current can then flow through the load connected, for example, between the points A₁ and C₁ when the switch member S₁₂ is switched on. Switching off of S₁₂ then results in the capacitor C₁₁ being discharged in the same manner as described above for the capacitor C₂. The energy stored in the capacitor C₁₁ arrives in the capacitor C₃, as a result of which the switch member S₁₁ will not be loaded by this discharging current during switching on thereof. The switch member S₁₂ is protected by its discharge capacitor C₁₂ from excessive voltage increase.
As evident from the above, two separate circuits are simultaneously active during switching off of a switch member. For the case described in Figures 8-10 the situation is that, when the switch member S₁ is switched off, the snubber circuit of S₁ is active, i.e. the diode D₁ and capacitor C₁, and that the capacitor C₂ of the snubber circuit of S₂ is active at the same time via the clipping circuit formed by the capacitor C₄ and the diode D₄. The capacitors C₁ and C₂ are because of this in parallel. The value of the separate capacitors can therefore in principle be chosen to be smaller, with the resultant smaller stray self-inductance (smaller dimensions). This may be referred to, as it were, as a primary and secondary switch snubbering. The primary switch snubber D₁, C₁ is very low in stray inductance and therefore limits V_DP (Figure 3) and thus the switch losses in the switch member. The secondary switch snubber C₄, D₄, C₂ assists in limiting the voltage increase after the voltage peak V_DP and vice versa.
In relation to the simple clipping circuit shown in Figure 5, the circuit according to the invention has the advantage that already the current flows through one branch of the whole clipping circuit, namely C₄ and D₄, before clipping off the voltage, as a result of which this clipping circuit will show less voltage overshoot than the clipping circuit according to Figure 5 because no instantaneous current can flow through this clipping circuit due to the stray self-inductance.
In the three-phase embodiment of the snubber circuit according to the invention, shown in Figure 11, the capacitors C₃ and C₄ are used jointly for the whole circuit. The energy discharge means are also only single-fold in embodiment, just like the current increase-limiting coil(s) L₁, L₂. It is of course possible to design the capacitors C₃ and C₄ separately for each circuit, while the advantage still remains that the energy discharge means can be used jointly for all said capacitors.
Figure 12 shows the snubber circuit according to the invention as applied in a so-called inverter, which can accept three potentials at its output C, namely positive, zero and negative. The connection shown in fact consists of two series-connected symmetrical snubber circuits according to Figure 6, the direct connection between the diodes D₁₂, D₂₁ and the switch members S₁₂, S₂₁ being interrupted, however. Both between the connecting point of the diodes D₁₂, D₂₁ and the connecting point of the capacitors C₁₁, C₁₂ as well as between the connecting point of the capacitors C₂₁, C₂₂ there is connected, a diode D₉ and Dʹ₉ respectively in a reverse direction relative to the terminal for direct voltage. The output of the circuit is formed by the connecting point C of the switch members S₁₂ and S₂₁. The output becomes positive when S₁₁ and S₁₂ are switched on, the output becomes zero when S₁₂ and S₂₁ are switched on, S₁₁ and S₂₂ being switched off, and the output becomes negative when S₂₁ and S₂₂ are switched on.
The switch snubber for S₁₁ is formed by D₁₁, C₁₁, the switch snubber for S₁₂ is formed by D₂₁, C₂₁, the switch snubber for S₂₁ is formed by D₁₂, C₁₂ and the switch snubber for S₂₂ is formed by D₂₂, C₂₂. The circuit operates as follows.
Assume that the switch members S₁₁ and S₁₂ are switched on so that the output C carried a positive potential. The capacitors C₁₁ and C₂₁ are then discharged and the capacitors C₁₂ and C₂₂ are charged up to U_c. During switching over the output C from a positive potential to a zero potential, S₁₁ is switched off and S₁₂ remains switched on. The current flowing through S₁₁ is taken over by its snubber circuit D₁, C₁₁ with parallel-connected circuits formed by C₁₄-D₁₄-C₁₂. As explained above, the capacitor C₁₁ is charged up to U_c and the capacitor C₁₂ is discharged to zero volt. The voltage over S₁₁ is limited by its clipping circuit consisting of C₁₄-D₁₄-D₁₂ parallel with the clipping circuit D₁₁-D₁₃-C₁₃ to the voltage U_c. The current through the load then flows from the zero terminal via the diode D₉ and S₁₂. S₂₁ is subsequently switched on, so that the current can flow from and to the 0-terminal.
During switching over of the output C from zero potential to a starting level with a negative potential, S₁₂ is switched off. If there is still current flowing through S₁₂ this current may flow further via the circuit formed by D₂₁-C₂₁-D₂₅ with the circuit, connected in parallel there to, formed by C₂₄-D₂₄-C₂₂-D₂₅. During this process the capacitor C₂₁ is charged up to a voltage U_c and the capacitor C₂₂ is discharged to 0 volt. On attainment of this situation the parallel clipping circuit C₂₄-D₂₄-D₂₂-D₂₆-D₂₅ and D₂₁-D₂₃-C₂₃-D₂₆-D₂₅ respectively become operative so that the voltage over the switched-off switch member S₁₂ is limited to the voltage U_c. S₂₂ can then be switched on.
If, during switching off of S₁₂ no current were to flow therethrough, the above described process in relation to opening of S₁₂ will not take place. During switching on of S₂₂ a resonant current will then be produced via the circuit formed by (D₂₁-C₂₁-S₂₂-L₂)//(C₂₄-D₂₄-C₂₂-S₂₂-L₂). As a result the capacitor C₂₁ will be charged up to a voltage U_c and the capacitor C₂₂ will be discharged to 0 volt.
The operation of the circuit during switching off of the other switch members takes place in a similar manner and can easily be deduced from the above. The above description shows that the snubber circuit for providing an output voltage at three levels essentially operates in the same way as in the circuit shown in Figure 6 in which the output can assume only two voltage levels. In this circuit a primary and a secondary snubber circuit may also always be allotted and two circuits always work in parallel during the protection of the switch members. As shown in figure 12 by dotted lines, a current increase-limiting coil L₄ can of course also be incorporated in the neutral line. It is furthermore possible to design the circuit shown in figure 12 to be multi-phase, such as the three-phase circuit shown, for example in figure 11. The means for discharging the energy may be connected, in a similar manner as shown, for example, in Figure 7, to the respective further capacitors C₁₄, C₁₃; C₂₄,C₂₃. In the said three-phase embodiment these further capacitors can be designed jointly for all phases.

Claims

Symmetrical electrical snubber circuit for protecting at least two series-connected switch members (S₁; S₂) of which the ends which are not connected to each other form a positive (A) and a negative (B) terminal, respectively, for connecting, via at least one coil (L₁; L₂), a direct voltage (U_DC), at least one first element (D₅; D₆) having diode action connected in the reverse direction and a series circuit of at least one capacitor (C₁; C₂) and at least one second element (D₁; D₂) having diode action connected in the forward direction relative to the terminals (A; B) for direct voltage being connected in parallel to each of the switch members (S₁; S₂) respectively, said at least one capacitor (C₁; C₂) of each series circuit being connected to the mutual connection point (C) of the switch members (S₁; S₂), and means (ET) for discharging the energy stored in the capacitors (C₁; C₂) being provided, characterized by being provided for each switch member (S₁; S₂) a further series circuit of at least one further capacitor (C₃; C₄) and at least one third element (D₃; D₄) having diode action connected in the forward direction relative to the terminals (A; B) for direct voltage, and being connected, respectively, between the connecting point of said at least one capacitor (C₁; C₂) and said at least one second element (D₁; D₂) having diode action of the series circuit associated with the one switch member (S₁; S₂) and the terminal (B; A) for direct voltage associated with the other switch member (S₂; S₁), all this in a manner such that a snubber circuit which is as low in stray inductance as possible is obtained, and wherein the means (ET) for discharging the stored energy being connected to the respective further capacitors (C₃; C₄).
Symmetrical electrical snubber circuit comprising a first and second series-connected symmetrical snubber circuit according to claim 1, each having at least two series-connected switch members (S₁₁, S₁₂; S₂₁, S₂₂), wherein the negative terminal (B,) for direct voltage of the first symmetrical snubber circuit and the positive terminal (A₂) for direct voltage of the second symmetrical snubber circuit being connected to each other and forming a zero potential terminal (O) for the direct voltage (½U_DC; ½U_DC) to be applied, the direct connection between this zero potential terminal (O) and the mutually connected ends (C) of the switch members (S₁₂; S₂₁) of said first and second symmetrical snubber circuit being interrupted and the respective first elements (D₁₅, D₁₆; D₂₅, D₂₆) having diode action being connected parallel to the switch members (S₁₁, S₁₂; S₂₁,, S₂₂) respectively, and wherein at least one element (D₉; D' 9) having diode action in the reverse direction relative to the terminals (A₁, B₁; A₂, B₂) for direct voltage being connected between the said zero potential terminal (O) and the mutual connecting points of the at least two series-connected switch members (S₁₁, S₁₂, S₂₁, S₂₂) of said first and second symmetrical snubber circuit.
Symmetrical electrical snubber circuit according to claim 1 or 2, comprising several parallel circuits of at least two series-connected switch members (S₁₁, S₁₂; S₂₁, S₂₂, S₃₁, S₃₂), wherein the further capacitors (C₃, C₄; C₁₃, C₁₄; C₂₃, C₂₄) are designed for all the circuits jointly and are connected at one end to a terminal (A₁, A₂, A₃; B₁, B₂, B₃) for direct voltage and at the other end to the associated corresponding third elements (D₁₃, D₂₃, D₃₃; D₁₄, D₂₄, D₃₄) having diode action.
Symmetrical electrical snubber circuit according to claim 1, 2 or 3, wherein the respective switch members (S₁, S₂; S₁₁-S₂₂; S₁₁-S₃₂) consist of a group of several parallel switch members.
Symmetrical electrical snubber circuit according to claim 1, 2, 3 or 4, wherein the switch members (S₁, S₂; S₁₁, S₁₂, S₂₁; S₂₂, S₃₁, S₃₂) of a circuit are designed as two-way switches.
Symmetrical electrical snubber circuit according to claim 1, 2, 3, 4 or 5, wherein the switch members (S₁, S₂; S₁₁-S₂₂; S₁₁-S₃₂) are controlled semiconductor elements.
Symmetrical electrical snubber circuit according to claim 6, wherein the semiconductor elements are GTO thyristors.